Dissolution of ammonium nitrate in water is what type of enthalpy change?

• A. Endothermic
• B. Exothermic
• C. No heat change
• D. Depends on temperature

Answer: (A)

Dissolution involves two competing energetic steps: breaking apart the solid lattice of the salt (which requires energy) and hydrating the ions in water (which releases energy). For ammonium nitrate, the energy needed to separate the ions in the lattice is larger than the energy released when water surrounds and stabilizes those ions. The net result is that the process absorbs heat from the surroundings, so the enthalpy change is positive. That’s why dissolving ammonium nitrate in water makes the solution feel cold. The other possibilities would imply no heat change or heat release, which doesn’t match the observed cooling. While some dissolutions can depend on temperature, ammonium nitrate’s dissolution remains endothermic across typical conditions, so the correct description is endothermic.